Electroplating on aluminum



Patented Apr. 27, 1954 to- Aluminum Company o=America, Pittsburgh, Pa.,a corporation of Pennsylvania No Drawing. Applicationseptember 23;1949.,

Serial No. 117,514.

43 Claims. (01. 204 33) This invention relates to electroplating onaluminum, with special reference to the zinc immersion procedure forpreparing aluminum articles for electroplating. As used herein, the termaluminum includes pure aluminum, commercial aluminum containing theusual impurities, and aluminum base alloys.

Electroplated aluminum articles have wide utility for variousapplications where a pleasing, decorative appearance is desired or whereother surface characteristics of electroplated articles are desired. Oneof the most satisfactory pros esses for plating aluminum articles startswith the zinc immersion procedure, in which procedure the aluminumarticles are immersed in an alkaline zincaterbath prior toelectroplating and thus acquire. a zinc coating over which electroplatesmay be deposited. The zinc coated aluminum articles may then beelectroplated with various metals and by various methods. However, evenwhen the zinc immersion procedure is used with very careful control ofall preparatory and plating steps, it isnot always possible to producearticles having strongly adherent electroplates and high resistance tocorrosion.

It is, therefore, an object of this invention to improve thezincimmersion procedure as part of a process of electroplating aluminumarticles, so that the electroplates show better adherence to thealuminum and the electroplated articles exhibit improved resistance tocorrosion. Other objects of the invention aretoimprove-the alkalinezincate bath used in the zinc immersion step and to improve the zinccoating produced thereby.

These and other objects and advantages of the invention will be apparentin the following de scription of the invention.

A typical zinc immersion procedure requires performance of the followingsteps in the preparation of the aluminum articles for electroplating:

. Buff (if the nature of the surface permits) Solvent clean Alkalineclean and water rinse Hot sulfuric acid dip (for some alloys) and waterrinse Cold nitric acid dip and water rinse Zinc immersion dip (zincatebath) and Water rinse After these steps are performed, variouselectroplates are produced on the aluminum articles by known methods. I

During the zinc immersion step above described,

a thin zinc deposit is formed on the surface of '2 the aluminum articleby immersing the aluminum article in an aqueous alkaline solution ofalkali metal zincate.

The zinc immersion step per se comprises immersing the aluminum articlefor about to 3 minutes, at room temperature, in an aqueous solution ofalkali metal zincate, which may be made up with about 100 grams of zincoxide and about 400 grams of caustic soda per liter of bath, the balancebeing substantially all water. Of course, the zincate bath may varywidely in the proportions and concentrations of its components. Theamount of zinc oxide may be much lower than 100' grams per liter ofbath, but the ratio of causitcsoda tozinc oxide should be between about3 1- to 8:1. Further the bath may be made up with equivalent amounts ofother zinc salts and other caustic alkalis; Various electroplatingpractices may be employed after the zinc immersion step to produce thetype of electroplate desired.

However, I have found that the provision, in any of the-above-describedalkaline zincate baths, of small amounts of at least one of the hydrousoxides of iron, cobalt and nickel (which form negative inorganic colloidsols in the alkaline bath) is of material advantage. Such an additionleads to the production of a zincdeposit on the aluminum surfacewhi-chisuniform, dense and adherent. This deposit permits the subsequentlyformed electroplate layers to build up smoothly and uniformly. In fact,electroplates applied over this zinc deposit are firmly adherent; andthe electroplated articles are, in general, more resistant to corrosion.

The hydrous oxides of the iron group are conveniently provided in thezincate bath by additions of salts of the iron group metals, such asferric chloride, cobalt chloride or sulfate, and nickel sulfate,preferably with additions of at least one protective colloid-agent, suchas tartrate, tannate, sugars (e. g. sucrose or glucose) glycerin,mannite or the like. The amount of iron group metal in the bath shouldbe about 0.1

to 1.5 grams (preferably about 0.2 gram) per liter.

of the bath. These indicated amounts of iron may be supplied byabout 0.5to 7.5 grams (preferably 1 gram) of ferric chloride per liter of thebath. Smaller amounts of iron or other iron group metals do not seem tobe elfective, and larger amounts tend to cause blistering ofelectroplates applied over the zinc deposit. The protective agents maybe employed in various amounts, usually at least about 5, and up toabout 50, grams per gram of metal from the iron group, although largeramounts are not usually harmful and may sometimes be desired. Whenferric chloride and tartrate are added, for example, the amount oftartrate may be about 1 to 50 grams per gram of ferric chloride.

By way of example, my improved alkaline zincate bath may be made up withwater and the following chemicals (grams per liter of bath):

ZnO 10,0 NaOH 400 FeCl3.6H2O l C4H40s= 5 By employing an alkalinezincate bath of the type described which has been improved by theprovision therein of a metal from the iron group, in the amountsindicated above, I am able to produce zinc deposits and subsequentelectroplates on aluminum articles which are smooth and adherent to thealuminum base, and more resistant to corrosion than aluminum articlesprepared by the zinc immersion procedure without an addition of the irongroup metal to the zincate bath.

In addition to the improvements above described, I have also found thatfurther improvements can be obtained when a double zinc immersionprocedure is employed. That is, after the zinc deposit is formed inaccordance with the procedure just described, the zinc deposit isstripped. This may conveniently be done by an immersion in nitric acid.Then the article is again immersed in the improved alkaline zincatebath. This procedure has the advantage of reducing the number of, oreliminating, the cleaning and preparatory steps ordinarily requiredprior to immersion in the zincate bath, while providing a uniformlyactive aluminum surface for reception of the last zinc deposit. The lastzinc deposit will be a very thin zinc deposit, which is continuous andsmooth, a type of zinc deposit that which I have found to be highlydesirable. Further, I have been consistently able to produce, over sucha zinc deposit electroplated aluminum articles having goodelectroplate-adherence characteristics and greatly improved resistanceto corrosion. These results I believe to be dependent upon theproduction of a uniformly clean and chemically active aluminum surfacecondition by the first treatment in the improved zincate bath, upon theproduction upon that surface of a uniformly thin, protective deposit ofzinc by the last treatment in the improved zincate bath, and finallyupon the production of smooth electroplates over a thin smooth zincdeposit.

I claim:

1. In a process for electroplating an aluminum article wherein thearticle is provided with a zinc deposit by immersion in an aqueousalkaline zincate bath and thereafter electroplated over said deposit,the improvement therein which comprises employing in said zincate bathat least one of the hydrous oxides of metals from the iron group, theamount of iron group metal provided being about 0.1 to 1.5 grams perliter of bath.

2. In a process according to claim 1, additionally employing in saidzincate bath at least about 5 grams of tartrate per gram of metal fromthe iron group.

3. In a process according to claim 1, the additional improvement thereinwhich comprises providing a preliminary zinc deposit by immersion insaid alkaline zincate bath and stripping the zinc deposit first formedin said alkaline zincate bath prior to providing the zinc deposit overwhich the electroplate is produced.

'4. In a process for electroplating an aluminum article wherein thearticle is provided with a zinc deposit by immersion in an aqueousalkaline zincate bath and thereafter electroplated over said deposit,the improvement therein which comprises employing in said zincate bathabout 0.5 to 7.5 grams of ferric chloride per liter of bath and about 1to 50 grams of tartrate per gram of ferric chloride so employed.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,045,713 Marion Nov. 26, 1912 1,372,290 Hurley Mar. 22, 19211,627,900 Hewitson May 10, 1927 2,142,564 Korpiun Jan. 3, 1939 2,377,605Blackmun June 5, 1945 2,418,265 Korpiun Apr. 1, 1947 2,485,182 ArentOct. 18, 1949 2,496,845 Balden Feb. 7, 1950 FOREIGN PATENTS NumberCountry Date 633,091 Germany Oct. 25, 1932 OTHER REFERENCES MetalFinishing, Feb. 1949, pages 51-54.

1. IN A PROCESS FOR ELECTROPLATING AN ALUMINUM ARTICLE WHEREIN THEARTICEL IS PROVIDED WITH A ZINC DEPOSIT BY IMMERSION IN AN AQUEOUSALKALINE ZINCATE BATH AND THEREAFTER ELECTROPLATED OVER SAID DEPOSIT,THE IMPROVEMENT THEREIN WHICH COMPRISES EMPLOYING IN SAID ZINCATE BATHAT LEAST ONE OF THE HYDROUS OXIDES OF METALS FROM THE IRON GROUP, THEAMOUNT OF IRON GROUP METAL PROVIDED BEING ABOUT 0.1 TO 1.5 GRAMS PERLITER OF BATH.